This invention relates generally to lighting techniques. More specifically, embodiments of the invention include techniques for combining one or more colored light emitting diodes (LEDs), such as violet, blue, blue and yellow, or blue and green, fabricated on bulk semipolar or nonpolar materials with use of materials such as phosphors, which emit light. In other embodiments, the entities may be also any wavelength conversion material(s) or energy conversion material(s), or the like. Herein the term “phosphor” is used to include all such types of materials. Merely by way of example, the invention can be applied to applications such as white lighting, multi-colored lighting, general illumination, decorative lighting, automotive and aircraft lamps, street lights, lighting for plant growth, indicator lights, lighting for flat panel displays, other optoelectronic devices, and the like.
In the late 1800's, Thomas Edison invented the light bulb. The conventional light bulb, commonly called the “Edison bulb,” has been used for over one hundred years. The conventional light bulb uses a tungsten filament enclosed in a glass bulb sealed in a base, which is screwed into a socket. The socket is coupled to an AC power or DC power source. The conventional light bulb can be found commonly in houses, buildings, and outdoor lightings, and other areas requiring light. Unfortunately, drawbacks exist with the conventional Edison light bulb. In particular, more than 90% of the energy used for the conventional light bulb dissipates as thermal energy. Additionally, the conventional light bulb routinely fails, often due to thermal expansion and contraction, of the filament element.
To overcome some of the drawbacks of the conventional light bulb, fluorescent lighting has been developed. Fluorescent lighting uses an optically clear tube structure filled with a halogen gas and, which typically also contains mercury. A pair of electrodes is coupled between the halogen gas and couples to an alternating power source through a ballast. Once the gas has been excited, it discharges to emit light. Typically, the optically clear tube is coated with phosphors, which are excited by the light. Many building structures use fluorescent lighting and, more recently, fluorescent lighting has been fitted onto a base structure, which couples into a standard socket.
Solid state lighting techniques have also been used. Solid state lighting relies upon semiconductor materials to produce light emitting diodes, commonly called LEDs. At first, red LEDs were demonstrated and introduced into commerce. Red LEDs use Aluminum Indium Gallium Phosphide or AlInGaP semiconductor materials. Most recently, Shuji Nakamura pioneered the use of InGaN materials to produce LEDs emitting light in the blue color range for blue LEDs. The blue colored LEDs led to innovations such as solid state white lighting, the blue laser diode, which in turn enabled the Blu-Ray™ (trademark of the Blu-Ray Disc Association) DVD player, and other developments. Other colored LEDs have also been proposed.
High intensity UV, blue, and green LEDs based on GaN have been proposed and even demonstrated with some success. Efficiencies have typically been highest in the UV-violet, dropping off as the emission wavelength increases to blue or green. Unfortunately, achieving high intensity, high-efficiency GaN-based green LEDs has been particularly problematic. The performance of optoelectronic devices fabricated on conventional c-plane GaN suffer from strong internal polarization fields, which spatially separate the electron and hole wave functions and lead to poor radiative recombination efficiency. Since this phenomenon becomes more pronounced in InGaN layers with increased indium content for increased wavelength emission, extending the performance of UV or blue GaN-based LEDs to the blue-green or green regime has been difficult. Furthermore, since increased indium content films often require reduced growth temperature, the crystal quality of the InGaN films is degraded. The difficulty of achieving a high intensity green LED has lead scientists and engineers to the term “green gap” to describe the unavailability of such green LED. In addition, the light emission efficiency of typical GaN-based LEDs drops off significantly at higher current densities, as are required for general illumination applications, a phenomenon known as “roll-over.” Other limitations with blue LEDs using c-plane GaN exist. These limitations include poor yields, low efficiencies, and reliability issues. Although highly successful, solid state lighting techniques must be improved for full exploitation of their potential. These and other limitations may be described throughout the present specification and more particularly below.
Light emission from GaN-based LEDs is typically unpolarized. Some applications, for example, liquid crystal displays such as are employed in televisions, computer monitors, and other large-area displays, require that the light source be polarized. Polarization is typically achieved by employing a conventional polarizer together with an unpolarized light source. However, this approach wastes more than 50% of the energy of the light source, increasing cost and decreasing efficiency, and results in increased complexity. If the light source had a modest degree of polarization the polarizer could still be employed, but the efficiency would be greater because a smaller fraction of the emitted light would be wasted. If the polarization of the source were sufficiently high it may be possible to omit the polarizer altogether, increasing efficiency and decreasing cost and system complexity.